1. Field of the Invention
The present invention relates to an image processing device for compressing image data.
2. Description of the Related Art
In devices for handling image data, it is common practice to compress image data in order to reduce data transfer load and lower data storage requirements. In order to compress image data efficiently, it is important to select the compression method and compression parameters to match the characteristics of the image data. For example, in the case of an image having comparatively low frequency spatial frequency components, such as a natural image, a JPEG (Joint Picture Experts Group) compression method is applied, while run length compression is applied to sections having consecutive characters or the like.
Most images are not made up of sections having only one characteristic, such just natural images or just text, and are in fact normally a combination of natural images and text. For this reason, if the same compression is carried out for all of the data, compression efficiency is lowered and quality is degraded. For example, if image data containing natural images and text is compressed using JPEG, it is possible to carry out appropriate compression for the natural image section, but the quality of the text section is degraded and compression efficiency is lowered.
There are therefore techniques to segment image data into natural image section planes and text section planes and apply appropriate compression, for each plane. In this case, segmentation into planes discriminates text regions using edge detection or presence or absence of restricted colour. Examples of such techniques are disclosed in Japanese Patent Laid-open No. Hei. 7-236062. A technique for segmenting into planes based on results of encoding is also disclosed in Japanese Patent Laid-open No. Hei. 11-168633.